Her story

Lenalee's past isn't told as much as Allen's in the manga. However, the fanbook Gray Ark reveals it for us. Have a summary:

Age ???

Her parents were killed by an akuma, and she has no memories of them. Her brother, Komui Lee, takes over the "parent" role for her sake, since she is the only relative he has.

Age 6

Deemed to be a Conformer of an Innocence, little Lenalee is separated from her only sibling and is taken to the Black Order by force. The first person she speaks with is the head nurse.

Age 8

Kanda Yuu enrolls at the Order. Because of his pretty and long hair, Lenalee mistakes him for a girl. She learns meditation from Kanda.

Age 9

Becomes ill at heart, and is confined.

Age 10

Reunites with Komui, who enrolls at the Black Order as an overseer. Jerry, the cook at the Order, teaches Lenalee a lady's manners.

Age 12

Lenalee begins helping out in the Science department. She has become her brother's assistant before she even realizes it.

Age 14

Lavi and Bookman joins the Order. She stops Kanda, who draws his sword upon Lavi.

The next page of her story carries heavy spoilers up to the latest chapter of the manga, but this will help you understanding the site's contents. Proceed anyway?